Oldlae:
but fuel flow did not reportedly increase
No Oldlae, you are making an assumption.
Nowhere in the report does it say that the fuel flow did not increase. What it says is that "the fuel
metering valves on both engines correctly moved to then fully open position to schedule an increase in fuel flow." You are making the causal assumption that since this occurred, and the engine did not respond, then the increased fuel flow did not occur.
As far as I can tell, the AAIB used those words because it does not have the data to show whether an increased fuel flow occurred or not.
To put it another way, if the AAIB knew for a fact that the fuel flow did not increase, they would have said the fuel flow did not increase, but that is not what they said.
If Mr. Husted's theory is correct, maybe an increase in fuel flow did occur, but the engine did not respond because of core icing.